A technique called an imaging mass spectrometry is known as one of the techniques to observe the two-dimensional distribution of substances on a sample surface. With the imaging mass spectrometry, laser beam or primary ions serve as primary particles and are irradiated to a sample surface, and according to this irradiation, the ions emitted from the sample surface (secondary ions) are subjected to mass spectrometry. Matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOFMS) that combines time-of-flight mass spectrometry (TOFMS) with an ion source by matrix-assisted laser desorption/ionization (MALDI) is generally used as the device that employs a laser beam as primary particles.
The technique of using primary ions as primary particles is called Secondary Ion Mass Spectrometry (SIMS), and as the device therefore, Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) using a time-of-flight mass analyzer as the mass analyzer is well known. TOF-SIMS has a disadvantage in terms of sensitivity, but is excellent in spatial resolution compared to that of MALDI-TOFMS. For this reason, imaging mass spectrometry that uses TOF-SIMS has been widely used in surface analysis of products and industrial materials such as silicon wafer, and the like.
In the imaging mass spectrometry using TOF-SIMS, the following two imaging techniques are used to obtain the intensity distribution of various ions within a somewhat wide region on a sample (refer to Non-Patent Literature 1).
(1) Scanning-type imaging technique: a primary ion beam is focused to a fine diameter and irradiated on a sample surface, and the sample surface is scanned two dimensionally with the beam so as to move its radiation position on the sample surface. Subsequently, mass spectrometry of the ions emitted from the measurement point on the sample is performed for every measurement point, thereby collecting the mass spectral data on each measurement point within the predetermined region on the sample.(2) Projection-type imaging technique: The diameter (cross-sectional area) of the primary ion beam is widen to irradiate to the sample surface, and the ions emitted from the wider range on the sample to which the ion beam has been irradiated are guided to a two-dimensional detector so as maintained its two-dimensional positional relationship to be detected. In this way, the mass information and the positional information of the ions emitted from different measurement points on the sample can be obtained simultaneously.
In the scanning-type imaging technique of (1), mass spectrometry needs to be performed sequentially for each of the multiple measurement points, so the time required for measurement becomes long when the region to be measured on the sample is wide. On the contrary, in the projection-type imaging technique of (2), even when the region to be measured on the sample is wide, it is possible to obtain the mass spectral data on all measurement points within the region by one mass analysis. For this reason, this technique has a significant advantage of being able to yield a high measurement throughput due to its ability to shorten the time required for measurement compared to that with the scanning-type imaging technique.
With such TOF-SIMS, secondary ions fly from the sample by the irradiation of primary ions on the sample, and the time of flight is measured using the point at the time the ions start to fly as the starting point. For this reason, the variation of the times of occurrence of the secondary ions on the sample leads to the decrease in mass accuracy and mass resolution in the data obtained. Therefore, primary ions are generally pulsed to irradiate on the sample. This is the same as irradiating to the sample a pulsed laser beam in MALDI-TOFMS.
However, when the pulse width of the ion beam is made to be narrow in order to suppress the temporal spread of the primary ions, the amount of the primary ions becomes low, deteriorating the detection sensitivity. On the other hand, when the pulse width is made to be wider, the amount of the primary ions increases, widening the temporal spread. For these reasons, in either the scanning-type imaging technique or the projection-type imaging technique, it becomes necessary to increase the amount of ions irradiated to the sample as much as possible while suppressing its temporal spread.
In TOF-SIMS by the projection-type imaging technique, primary ions in a thin sheet form in their travelling direction is irradiated to the sample. In this case, when there is a deviation in the arrival time of the primary ions arrived at multiple measurement points on the sample, a variation in the time of occurrence of the secondary ions occurs in each measurement point. For this reason, in order to acquire an accurate mass spectrometric imaging image, not only it is necessary to suppress as much as possible the temporal spread of the primary ions at arrival at each measurement point, but it is also necessary to suppress the deviation of the arrival time of ions among different measurement points within the region subjected to the measurement. That is, the primary ions in a thin sheet form need to simultaneously arrive at the region subjected to the measurement on the sample.
One of the factors which cause the deviation in the arrival time of the primary ions among the measurement points is that the optical path length of each ion is different according to the location within the cross section of one ion packet. This difference is due to that the primary ion beam which spreads spatially is irradiated to the sample surface from the diagonal direction. In order to solve this problem, a technique of deflecting the trajectory of the primary ions is employed in the TOF-SIMS disclosed in Patent Literature 1. According to this technique, it is able to suppress the variation in the arrival time of the primary ions to the sample surface among different measurement points occurred due to the difference in the optical path length of the primary ions from the primary ion generating part until the sample surface.
However, the technique described above, as described in paragraph (0045) of the Patent Literature 1, does not solve the problem of variation in the arrival time (that is, the temporal spread of the ion beam) caused by the spatial variation of the position existed in each ion at the time of irradiation from the primary ion generation unit. The temporal spread of the ion beam caused by such factor actually exits from hundreds of nsec to several usec. For this reason, even if the temporal variation due to the difference in the optical path length of the ion has been corrected, in order to obtain a sufficiently high mass resolution in TOFMS, it is difficult to fulfill the strict requirements of the pulse width of the primary ion beam to be about several nsec or less.
In order to realize a high mass resolution and high mass accuracy in TOF-SIMS using such a projection-type imaging technique, even when the positions of the ions in the travelling direction vary to a certain degree at each position within the cross section of the ion beam when the ions are emitted from the primary ion generation unit, it is important to sufficiently suppress the temporal spread of the ion beam at the time of irradiating the primary ions to the sample surface.
The problems described above apply not only to TOF-SIMS but also to various surface analyzer for detecting those other than ions, for example, light including x-rays, electrons, neutral particles, and the like, emitted from a sample according to the irradiation of ions to the surface of the sample.